General-purpose plastics are widely used for such products as require semi-permanency by virtue of their processability and chemical stability. In contrast to the semi-permanent products, plastic products such as disposable food vessels, waste envelopes, shock-absorbers for packages, etc, are short-lived. It is preferable in the view of preventing environmental pollution that the short-lived plastic products are degraded fast when they are discarded after use. Now, active research has been directed to the development of degradable plastics.
Depending on degradation mechanisms, degradable plastics are divided largely into biodegradable (Japanese Pat. Laid-Open Nos. Heisei 4-189822, Heisei 1-156319, Showa 59-213724, Showa 58-150525), biodeintegratible, and photodegradable ones. For the preparation of the biodegradable plastics, the following three-type materials are known to be used: microorganisms products, natural polymers, and microorganism-degradable synthetic polymers. Of the synthetic polymers, only aliphatic polyesters are completely degraded.
Polytetramethylene succinate, an aliphatic polyester, made mainly of succinic acid and tetramethylene glycol, is superior in thermal properties and is regarded as one of the most promising materials for industrialization. In addition to being expensive, however, aliphatic polyester shows faulty elongation and tear strength upon molding into film. These disadvantages incite an attempt to be made to modify the polyester with aliphatic carboxylic acids, such as adipic acid, or their derivatives, and with aliphaticalkylene glycols, such as ethylene glycol, or their derivatives, so as to give polyester compositions an improvement in film-moldability. However, most of these compositions are lower in melting point by 20-50.degree. C. and relatively poor in mechanical properties including tear strength, as compared with polytetramethylene succinate.
In Japanese Pat. Laid-Open Nos. Heisei 5-70577, 70578 and 70579 and U.S. Pat. Nos. 4,269,945 and 4,859,743, materials of isocyanate group are suggested to serve as chain extenders with the aid of which aliphatic polyesters with large molecular weights are prepared. After discard, these aliphatic polyesters are not completely decomposed in soil owing to the crosslink between main chains. Even if they are degraded, the isocyanate groups used remain, causing significant secondary soil pollution.
As a solution for improved biodegradability and mechanical properties in polyester, complexes of polyesters and biodegradable materials, for example, polyesters and natural materials, polyolefins and natural materials such as cellulose, starch, etc. (U.S. Pat. No. 4,337,181 and E.P. Nos. 400,531, 404,723 and 376,201), polyolefins and polyesters, and polyesters and polyesters, have been actively studied. The complexes have an advantage of being easily prepared using existing extruder technology by a conventional processes. However, since polyester materials and other materials are very poor in compatibility or affinity, the blending ratios therebetween are very limited. More than their limit causes the components to respectively aggregate, leading to non-uniform product quality and decreased mechanical properties.
Compared with aliphatic polyesters, aromatic polyesters such as polyethylene terephthalate, are low-priced and excellent in almost all properties including mechanical strength, thermal resistance, electrical insulation, etc., so they are widely used in fibers, films, and industrial materials. Aromatic polyesters does not show degradability at all nor are they themselves used as degradable materials.
With the aim of introducing such excellent properties of aromatic polyesters into degradable polyesters, aromatic polyesters and aliphatic polyesters were blended. Although the resulting blends are much improved in mechanical properties, phase separation occurs therebetween, so that the aliphatic polyesters only are degraded while the aromatic polyesters remain non-degraded.
It was reported in Journal of Applied Polymer Science, 26, 441, 1981) that aromatic/aliphatic polyesters are not degradable when the aromatic blocks in their intramolecular structures are long whereas they can be degraded when the aromatic blocks are short. The aromatic block in the aromatic/aliphatic polyester cannot be shortened by a simple blending technique. Random polymerization with component monomers makes the aromatic block as short as possible.
Since then, biodegradable copolyesters have been prepared by use of aliphatic monomers and aromatic monomers. They, except for those into which succinic acid, terephthalic acid and tetramethylene glycol are introduced, are insufficient in degradability, thermal properties mechanical properties, and cost.
Polyester's structures and physical properties, particularly, molecular weight distributions and mechanical properties, are greatly determined by the catalysts and monomers used upon its polymerization.
As for the catalysts, they are usually selected from the metal compounds of zirconium, potassium, antimony, titanium, germanium, tin, zinc, manganese and lead. It is well known that the kinds of the metals and their coordinated complexes give a great influence on the reaction rate, thermal properties, mechanical properties and molecular weight distribution of the polyester produced. Thus, it is very important to select appropriate catalysts for improving the reaction rate and mechanical properties of polyesters.
Usually, tin compounds, particularly, monobutyltin oxide or dibutyltinoxide, are widely used in the ester reaction of aliphatic and aromatic starting materials. The tin compounds are excellent in catalytic activity, but because the tin compounds are highly apt to be oxidized, a clouding phenomenon appears on the products when they are exposed to the air for a long period of time. So, their use, if possible, is restrained.
In preparing polyesters, a titanium compound, particularly, tetrabutyl titanate or titanium isopropoxide, is used as a polycondensation catalyst by virtue of its high catalytic activity. However, large amounts of these catalysts are required. Further, the resulting products are so poor in thermal stability that they are easily discolored. When raising the temperature of the polycondensation, a yellowing phenomenon is obviously surfaced on the polyester products.
In order to solve the above-mentioned problems, a great deal of research has been made on catalysts and additives. For example, for the purpose of time reduction and color improvement, there are used silicon compounds and titanium compounds (U.S. Pat. No. 3,927,052), antimony trioxide, cobalt compounds and phosphorous compounds (Japanese Pat. Laid-Open Publication No. Sho. 53-51295), antimony compounds and organic acids (Japanese Pat. Laid-Open Publication No. Sho. 60-166320), antimony compounds, cobalt compound and alkaline metal compounds (Japanese Pat. Laid-Open No. Sho. 49-31317), antimony compounds, tin compounds, cobalt compounds, alkali, and phosphorous compounds (Japanese Pat. Laid-Open No. Sho. 62-265324). Most of these techniques, however, cannot reduce the reaction time in both ester reaction and polycondensation nor bring about a remarkable improvement in the color of the products.